Methods and apparatus for suspending vehicles

ABSTRACT

A method and apparatus for a shock absorber for a vehicle having a gas spring with first and second gas chambers, wherein the first chamber is utilized during a first travel portion of the shock absorber and the first and second chambers are both utilized during a second portion of travel. In one embodiment, a travel adjustment assembly is configured to selectively communicate a first gas chamber with a negative gas chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to methods and apparatusfor use in vehicle suspension. Particular embodiments of the inventionrelate to methods and apparatus useful for variable spring rate and/orvariable damping rate vehicle suspension.

2. Description of the Related Art

Vehicle suspension systems typically include a spring component orcomponents and a damping component or components. Frequently thosediscrete components are separately mounted on a vehicle. Traditionally,mechanical springs, such as metal leaf or helical springs, have beenused in conjunction with some type of viscous fluid based dampingmechanism mounted functionally in parallel. More recently, compressedgas acting over a piston area has replaced mechanical springs as thespring component in some contemporary suspension systems. Whilecompressed gas springs are usually lighter and more compact than theirmechanical counterparts, the compression and expansion curve andcorresponding spring rate, are not linear and become particularlyexponential beyond a mid range of gas compression.

As such, the force (corresponding to pressure acting on a given pistonarea) versus the linear travel or displacement of the air spring is notlinear. While a gas spring force curve approximates linearity during aninitial portion of travel, the last portion of travel is exponential.The shock absorber is therefore increasingly more rigid in the lastportion of its stroke.

Accordingly, there is a need for a shock absorber that uses a multiplevolume gas spring under a variety of loads and/or under a variety oftravel settings.

SUMMARY OF THE INVENTION

The present invention generally comprises a shock absorber for a vehiclehaving a gas spring with first and second gas chambers, wherein thefirst chamber is utilized during a first travel portion of the shockabsorber and the first and second chambers are both utilized during asecond portion of travel. In one embodiment, a travel adjustmentassembly is configured to selectively communicate a first gas chamberwith a negative gas chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a view of a telescopic, fork-type shock absorber.

FIG. 2A is a section view of an upper portion of a shock absorberaccording to one embodiment and FIG. 2B is the lower portion thereof.

FIG. 2C is a section view of the embodiment of FIGS. 2A, B in acompressed position.

FIG. 2D is a section view illustrating a feature utilizing a negativegas chamber.

FIG. 3A is a section view of an upper portion of a shock absorberaccording to another embodiment and FIG. 3B is the lower portionthereof.

FIG. 3C is a section view of the embodiment in a compressed position.

FIG. 4 is a section view of another embodiment, shown in an openposition.

DETAILED DESCRIPTION

In this disclosure the term “air” is used interchangeably with the term“gas” unless otherwise stated. Both terms generally indicate acompressible fluid. One embodiment comprises an air spring shockabsorber for a vehicle. In one embodiment the vehicle is a bicycle. Inone embodiment the shock absorber is a bicycle or motorcycle front forkleg. The terms “shock absorber” and “front/leg” will each include allterms and unless an embodiment is expressly excluded, embodiments hereofare equally applicable to all. The air spring is advantageous because itincludes at least two sequentially activated gas spring chambers thatoperate to increase the effective gas volume of the spring, at least onecommunication valve for opening a fluid flow path between the chambers,and a fill valve for selectively applying gas pressure within thechambers. In one embodiment, the fluid path between the chambers isopened using a mechanically actuated valve and in another embodiment, adiameter change or “bypass” type valve opens the fluid path between thechambers. In yet another embodiment, the fluid path is opened solely bygas pressure developed in the first chamber.

In one embodiment, the multiple gas chamber spring can further includeand operate in conjunction with a damper. In one embodiment the dampercomprises a viscous fluid that is isolated from the compressible springfluid. Such viscous fluid may be isolated or distanced from the fluidspring in various ways. In one embodiment, such isolation may befacilitated by placing the damper component in one leg of a fork and thespring component in the other. A relevant feature of the isolationmechanism is that the damping fluid and the spring fluid are notco-located in the same dynamic volume. By providing some degree ofisolation or separation between the damping and spring fluids, which areoften liquid and gas respectively, the formation of emulsion is avoidedor greatly delayed. In one embodiment, the combined versatility of anadjustable or “tunable” damper and the multi-chamber spring result in ashock absorber having a selectively variable force versus travel versusvelocity curve (e.g., 3-d surface as plotted). In yet other embodiments,the multi-chamber air spring is utilized to improve the overallperformance of a shock absorber having a damper with automatic and useradjustable “blow off” features.

A piston in cylinder type suspension gas spring preferably includesenough gas volume so that the gas compression curve, for a correspondingsuspension system, remains substantially linear over a portion of thestroke (e.g. first ⅔rds) of the suspension system. Because pressure dueto gas compression increases exponentially, simple gas springs, as apractical matter, have spring rates that are often too low over thefirst half of the stroke and too high over the second half. Because thespring rate is initially too low, the initial gas pressure in a gasspring shock absorber must be set high to yield a usable shock absorber(e.g. one that is not too soft). Unfortunately such a shock absorber, asit is compressed, becomes quickly very stiff and allows little “settle”or “sag” when the shock initially loaded. Because proper initial “sag”improves vehicle ride and handling, lack of proper “sag” can negativelyaffect handling characteristics of a vehicle. Embodiments describedherein extend the substantially linear portion of the spring rate curvebeyond that represented by a single chamber and therefore allow forhigher initial pressure settings without overly accelerating the onsetof unusable stiffness during compression.

As vehicle use becomes more extreme, there is a need for increasedsuspension stroke. With single chamber air springs good linear springrate and usable stroke typically makes up a little over half of themechanical stroke. Usually the greater the mechanical stroke, the longerthe suspension (telescopic) must be. In a single chamber air spring, thelength might increase 1.5 inches for every additional inch of usablelinear rate stroke. Many vehicles have suspension envelopes that do notnecessarily have the length required to accommodate the stroke requiredfor desired vehicle performance. The multi-chamber air spring allows forincreased usable stroke without as much increased overall length.Additionally, as the suspension requirements become more robust, thestrength of the suspension package must increase. Again, the vehicledesign envelope may not allow enough space for such an increase inconventional parallel dampers and springs. An integrated viscousdamper/spring assembly reduces space requirements. As suspension strokerequirements increase and the suspension systems become correspondinglylonger, it is desirable to have the characteristics of different gasspring volumes at different points in the stroke in order to maximizeapproximate linear and usable spring rates.

U.S. Pat. Nos. 6,105,988 and 6,311,962 show a structure of a gas springand damper assembly and U.S. Pat. No. 6,360,857 shows a structure of adamper having some adjustable features and each of those patents areincorporated herein, in their entirety by reference.

FIG. 1 is a perspective view of a telescopic, fork-type shock absorber20 as would be found on a two wheeled vehicle like a mountain bike or amotor cycle. In the fork shown, a lower portion 25 of each leg has atelescopic relationship with an upper portion 30 and the two movetowards one another as the shock absorber 20 operates. In a typicalexample, one of the fork legs includes a gas spring and the otherincludes a fluid damper.

FIG. 2A is a section view of an upper portion of a shock absorberaccording to one embodiment and FIG. 2B is the lower portion thereof.FIG. 2C is a section view of the embodiment of FIGS. 2A, B in acompressed position. For ease of description, an outer tube covering thelower portion of the fork (visible in FIG. 1) is not shown. As with theshock absorber of FIG. 1, a lower portion of the fork body 5 extendsinto an upper portion 10 as the shock absorber operates. Included is afill valve 171 located at an upper end of the leg and usable to fill afirst gas spring chamber 165 via an internal fluid path through an upperportion of the leg. The first gas spring chamber 165 is separated from asecond gas spring chamber 170 by a bulkhead 300.

Referring to the Figures, as the lower fork body 5 moves in compression,carrying a gas compression piston 160 correspondingly further into theupper tube 10, the volume of the first gas spring chamber 165 isreduced, thereby compressing or further compressing the gas in the firstgas spring chamber 165. In the embodiment shown, a communication valve185 is disposed at the upper end of a communication valve shaft 167. Thevalve, which is normally closed due to a biasing member or spring 168,opens upon the application of an axial force to shaft 167, to allowcommunication between the first gas spring chamber 165 and the secondgas spring chamber 170. In one embodiment the axial force is supplied bycontact with the moving compression piston 160. Valve 185, with its sealring (e.g. O-ring) blocking a fluid path through a valve seat 186, isshown in FIG. 2A.

In one embodiment, the gas pressure in the first gas spring chamber 165continues to increase until a top portion 161 of the gas compressionpiston 160 impinges upon the lower end of the communication valve shaft167. The force exerted by the gas compression piston 160 on the lowerend of the communication valve shaft 167 moves the valve 185 off of thevalve seat 186, thereby opening a fluid flow path 187 through valve seat186 and between the first gas spring chamber 165 and the second gasspring chamber 170. As a result of the fluid communication between thetwo gas spring chambers 165, 170, any pressure differential between thechambers equalizes. Additionally, the effective volume of the shockabsorber gas spring is increased over the volume of the first gas springchamber 165 by the volume of the second gas spring chamber 170.

In the air spring shock absorber disclosed herein there are severalparameters that can be varied in order to derive a preferred travelversus pressure (i.e. force) profile, or “spring rate” profile over therange of travel. Variables that may be selectively altered include:length and diameter of the first chamber 165, volume of the secondchamber 170, initial pressure state of the first chamber 165, initialpressure state of the second chamber 170, and length and/or position ofthe communication valve shaft 167. In one embodiment, piston areas of apressure divider and initial check valve spring load may be varied.

The initial pressure state and the diameter of the first chamber 165define the shape of the travel versus spring pressure profile for theshock absorber prior to opening the communication valve 185. Thelocation, along the travel, of the opening of the valve 185 determineswhen the spring force/travel curve of the first chamber alone is alteredand combined characteristically with the second, or additional,chamber(s). Preferably, the values chosen for those variables result ina substantially linear spring rate prior to, and following, fluidcommunication between the chambers 165, 170.

In one embodiment, the initial pressure in the second chamber 170 is setto equal a pre-calculated pressure in the first chamber 165corresponding to a point just before the gas compression piston 160contacts the lower end of the communication valve shaft 167. When thecommunication valve 185 is opened with such a second chamber 170pressure setting, there is no significant differential pressure, at thatpoint, between the first and second chambers 165, 170, and hence theforce versus travel curves before and after are blended together at thetransition. Further, there is no significant system pressure drop whenthe first and second chambers 165, 170 are fluidly communicated. The gasspring volume is increased by the amount of the second chamber 170 andthe spring rate is correspondingly decreased. However, the transitionfrom the spring rate associated with only the first chamber 165 to thespring rate associated with the combined chambers 165, 170 is relativelysmooth (in one embodiment to the point where the combined volumesproduce a spring rate approximating a constant).

Alternatively the initial pressure in the second chamber 170 may be setat the same pressure (and time) as the initial (fully extended) pressurein the first chamber 165. During an initial compression of the shockabsorber the volume of the first chamber 165 is reduced and the pressurein the first chamber 165 rises until the communication valve 185 isopened. Because the second chamber 170 pressure is still at its initial(and now lower) pressure setting, fluid flows from the first chamber165, through the communication valve 185 and into the second chamber 170when valve 185 is opened. The pressure in the now-combined first andsecond chambers 165, 170 equalizes at a pressure value somewhere betweenthe pre-communication first chamber pressure and the initial secondchamber pressure (the equalization pressure depends on the relativevolumes of the first and second chambers 165, 170 and the first chamberdisplacement that occurs prior to equalization). During subsequentcompression cycles of the shock absorber, the second chamber 170 retainsthe compression pressure of the first chamber 165 as a set point and nofurther equalization occurs upon opening of the communication valve 185.Optionally, a one-way valve (e.g., check valve not shown) is separatelyincluded between the chambers (see description of embodiment of FIG. 4)and permits communication from the second to the first chamber duringrebound. In this manner, compressed gas is allowed to escape from thesecond to the first chamber upon extension of the shock absorber,thereby resetting the second chamber to a lower pressure, the pressuredetermined by the characteristics of a spring-biased check valve (e.g.higher spring bias equals greater retained differential pressure wherelower bias equals more equalization), for example.

It may be desirable to select the point in the travel at which the first165 and second 170 chambers are communicated. In one embodiment, thecommunication valve shaft 167 is available in different lengths. A usermay install a longer length valve shaft 167 for communication earlier inthe shock compression stroke or a shorter length for communication laterin the shock stroke. In one embodiment the initial travel setting of thefork leg is adjustable hence the distance into the travel where thevalve shaft 167 is contacted.

In the embodiment shown in FIGS. 2A-C, the communication valve 185 andshaft 167 are not coaxial with the center line of the shock absorber,thereby allowing a travel adjust support shaft 155 to be coaxiallylocated. The travel adjust support shaft 155 includes control mechanismsthrough or around the shaft 155 for selectively adjusting and/orblocking valves or orifices of the air spring assembly to effect changesin travel setting (see description of embodiment of FIG. 2D). The traveladjust assembly is preferably mounted on a coaxial (with the fork leg)shaft to minimize any bending and binding of the piston assembly duringuse.

The air spring is intended in some embodiments to be utilized in a shockabsorber system that includes a damper and the operation of the firstand secondary chambers permit the damper to operate in its mosteffective way over the course of the shock's operation. For example, adamper is most effective during the linear part of the gas spring curvewhen, depending upon the speed at which the shock is operating, thedamper meters fluid from one side of a piston assembly to the other,effectively absorbing (ultimately converting it to heat and dissipatingit) energy. During the nonlinear (exponentially increasing springstiffness) part of the spring curve, the damper provides less orvirtually no damping action since the shock has become so stiff thatmovement of the shock is limited and the damper is unable to metersignificant fluid.

In one embodiment the fork includes a damper lock which substantiallyprevents fluid transfer from taking place within a portion of thedamper. The lock is configured so that the damper becomes substantiallyrigid when the fluid transfer path is blocked. Such a feature allows auser to selectively lock the fork into a substantially rigidconfiguration in order to minimize “pedal bob” or other vehicle powerdissipation due to unwanted fork compression under power. Even whenlocked there is the possibility that a disparity in the terrain willrequire activation of the shock to prevent damage to the shock and/orvehicle. For that reason the shock having a damper lock as described mayalso be equipped with a blow off feature. One such damper lock/blow offfeature is described in U.S. Pat. No, 7,163,222 which patent isincorporated herein by reference in its entirety. In one embodiment, thedual chamber air spring is used with a damper having an adjustableblow-off feature. The blow off feature is an automatic overridepermitting the damper in a “locked out” shock absorber to operate andmeter fluid if subjected to a rapid shock event, like a sudden, abruptbump in a road. With the dual gas chambers, a suddenly operated or“blown off” damper will be more likely to be operating in a linearportion of the spring curve.

In one embodiment, a travel adjust is intended to permit a user toadjust the length of the air spring stroke in addition to opening thesecond chamber. As shown in FIGS. 2B, C, in addition to the first andsecond gas spring chambers 165, 170, there is a negative gas chamber150. Pressure in the negative chamber 150 acts on surface areasgenerally opposed to those of the gas compression piston 160. In oneembodiment the surface areas of the negative piston and the gascompression piston 160 are equal and the otherwise unloaded fork springis at pressure equilibrium when the pressure in the (main) first gasspring chamber 165 is equal to the pressure in the negative gas chamber150. Depending on the relative surface areas of the negative piston andthe gas compression piston 160, the equilibrium pressures in the firstgas spring chamber 165 and the negative gas chamber 150 may vary (e.g.be unequal).

FIG. 2D illustrates one embodiment wherein shaft 155 includes aconcentrically mounted tube set 156, 158 therein, the tubes havingvarious apertures through their walls with seals positioned betweencertain of the apertures. The innermost tube 156 is axially moveablerelative to the other tubes and in one embodiment is adjustable by meansof a knob on top of the fork leg (not shown). The tube set, by means ofthe apertures is capable of communicating pressure from the main chamber165 to the negative chamber 150 via an inner flow path 410 of theinnermost tube 156 where the inner flow path 410 is in communication ata lower end thereof with the negative spring chamber 150. Depending onwhich apertures are adjusted to perform the communication function, andthe axial location of those apertures along the tube set, the upper tube10, and hardware attached to it, will settle in equilibrium at differentaxial positions (corresponding to different travel settings) relative tothe lower body 5 and the lower fork leg (not shown).

In the example illustrated by FIG. 2D, apertures 157 in innermost tube156 are located just above an O-ring 154 that seals an annulus betweenthe tubes. In this setting, gas from main chamber 165 enters outer tube155 at apertures 159 and travels in an annular space between the outer155 and intermediate 158 tubes until it enters intermediate tube throughapertures 153. Thereafter, the gas enters the innermost tube throughapertures 157. Once in the innermost tube, the gas travels down to thenegative chamber 150 where it equalizes the pressure therein with thatof the main chamber 165. In another setting, the apertures of innermosttube are located just below the O-ring. In this position gas reachingthe apertures 157 of innermost tube 156 can only travel from a lower setof apertures 159 on the outer tube 155. Once the piston, with its seals,moves across the apertures 159, equalization is no longer possible.

FIG. 3A is a section view of an upper portion of another embodiment, andFIG. 3B is a lower portion thereof. FIG. 3C is a section view of theembodiment in a compressed position. Shown in FIGS. 3A-3C is anembodiment having travel adjust as described above (and shown) and aby-pass valve type of communication mechanism between a primary gaschamber and a secondary gas chamber. Like the embodiment of FIGS. 2A-C,an outer tube housing the lower body 5 of the fork is not shown. FIG. 3Bshows the lower body 5 carrying the gas compression piston 160,telescopically positioned within the upper fork tube 10. Compression ofthe fork leg causes the body 5 to move further up into the upper tube 10thereby compressing the gas in main chamber 165. The upper tube includestwo differing internal diameters: the primary diameter 230 and a by-passdiameter 240. The by-pass diameter 240 is located at an axial locationwithin the upper tube 10 corresponding to a point during compressionwhere the gas compression curve in the tube (due to remaining volume) isbeginning to get steep relative to incremental increases in compressivetravel (e.g., the diameter 240 may begin at the 50% compression point).

The gas compression piston 160 includes a housing 160′ that forms aportion of the enclosure isolating the secondary gas chamber 170 (theremainder of the enclosure is formed by diameter 230) from the mainchamber 165. When the seal of piston 160 enters the by-pass diameter 240(as shown in FIG. 3C), the additional volume of the secondary chamber170 is comingled with the main chamber 165 and a corresponding drop ingas compression rate can occur. Optionally (and as shown), the by-passdiameter 240 reduces closer to the top of the main chamber 165 andduring extensive compression the seal of the upper piston 160 willengage upper diameter 230, thereby re-isolating the main chamber 165from the secondary chamber 170. Such option provides increasedcompression rates near the end of the stroke to avoid harsh suspensionbottom out (i.e. gas cushion provided).

In summary, during a first portion of a compression stroke, the mainchamber 165 includes that portion of upper tube 10 above piston 160,which is sealed against a wall of tube 10 via O-rings. However, when thepiston 160 enters the bypass diameter 240, the second chamber 170, whichis a fixed volume chamber, is exposed to the main chamber 165.

FIG. 4 is a section view of another embodiment, shown in an openposition. In the embodiment of FIG. 4, operation of the shock absorberis pressure, rather than position sensitive. FIG. 4 illustrates thedevice in an “open” position, permitting communication between twochambers 165 and 170 as shown by arrow 187. The figure shows the upperportion 10 of a fork leg corresponding generally to FIGS. 3A and B. Asis typical, the fork includes a threaded cap member 320 and a valve 325for filling main chamber 165. As shown, the secondary gas chamber 170 isincluded in a generally annular space within the upper tube 10 and abovea sealed bulkhead 300. A valve 305 is biased normally closed by a spring310. At a lower end of the valve a piston surface 312 is formed. Whenpressure in the main chamber 165 reaches a preset valve “cracking”pressure, the volume of the secondary chamber 170 is comingled with thatof the main chamber 165. The compression placed on spring 310 (and acingagainst the pressure in chamber 165 is preset by a user-operableadjustment rod 315 which is threaded and designed to move axiallydownward in order to place the spring 310 in compression, therebyraising the pressure at which the valve operates. In order to permitpressurized gas to return to the main chamber 165, the assembly includesa second, one way valve 350 with a second spring 355. When valve member350 is displaced by pressure it opens permitting gas to flow in to themain chamber 165.

The fill valve and shock absorber/fork shown and disclosed in theFigures herein include o-ring seals as shown and where appropriate. Anysuitable seals may be used and seals may be used where not shown oromitted even though shown in any case as appropriate for the channelingand retention of pressurized fluids.

The dual chamber arrangement described herein permits the linear portionof the spring curve to continue through a greater range of shock traveland delays the less desirable non-linear portion, thereby resulting inan improved overall shock absorber function including damping.

While the invention has been described with only a first and secondchambers, the invention can be used with three or more separatechambers, each designed to operate together in a sequential fashion. Forexample, by arranging valves in a sequential manner, a first auxiliarychamber can be utilized based upon a first pressure or position of thecomponents in a compression stroke. Thereafter, based upon a secondpressure or position, a third chamber can be opened in a manner that allthree chambers operate as one.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the scope thereof, and the scope thereof is determined bythe claims that follow.

The invention claimed is:
 1. A shock absorber for a vehicle comprising:a gas spring having first and second gas chambers, wherein the first gaschamber is utilized during a first travel portion of a piston and thefirst and second gas chambers are both utilized during a second travelportion of the piston; a bulkhead separating the first and second gaschamber, the bulkhead having a valve with a valve shaft biased intosealing engagement with a valve seat in the bulkhead, the valve shaft atleast partially extending into the first gas chamber, wherein the pistonis constructed and arranged to contact the valve shaft at apredetermined position of the first travel portion, thereby opening thevalve and utilizing both the first and second gas chamber; and a traveladjustment assembly configured to selectively communicate the first gaschamber with a negative gas chamber, the travel adjustment assemblyincluding an inner tube at least partially disposed within an outertube, the piston movable relative to the outer tube and the inner tubemovable relative to the outer tube, the inner and outer tubes eachhaving at least one aperture in a wall thereof forming a communicationpath between the first gas chamber and the negative gas chamber atpredetermined positions of the piston in a compression stroke of shockabsorber.
 2. The shock absorber of claim 1, wherein the shock absorberis disposed in a leg of a fork.
 3. The shock absorber of claim 2,wherein a second leg of the fork comprises a fluid damper.
 4. The shockabsorber of claim 1, wherein utilization of the second gas chamber isbrought about by a predetermined pressure developed in the first gaschamber.
 5. The shock absorber of claim 1 wherein, at an end of thefirst travel portion, gas pressure is substantially the same in thefirst and second gas chambers.
 6. The shock absorber of claim 1 wherein,at a beginning of the first travel portion, gas pressure issubstantially the same in the first and second gas chambers.
 7. Theshock absorber of claim 1, wherein the valve shaft extends non-coaxiallyinto the first gas chamber.
 8. The shock absorber of claim 7, wherein aninitial position of the valve shaft relative to the piston is useradjustable.
 9. The shock absorber of claim 1, wherein the piston isconstructed and arranged to contact the valve shaft in the compressionstroke of the shock absorber.
 10. The shock absorber of claim 1, whereina length of the first travel portion is adjustable.
 11. The shockabsorber of claim 10, wherein the length of the first travel portion isadjusted by changing a length of the valve shaft.
 12. The shock absorberof claim 1, wherein, the first gas chamber is located on a first side ofthe piston and the negative gas chamber is located on a second side ofthe piston.
 13. The shock absorber of claim 1, wherein, in thecompression stroke of the shock absorber, the first and second gaschambers are located above the piston and the negative gas chamber isbelow the piston.
 14. The shock absorber of claim 1, wherein the outertube and the inner tube are concentric tubular members.
 15. The shockabsorber of claim 1, wherein the piston is constructed and arranged tomove the valve shaft into the second gas chamber.
 16. The shock absorberof claim 1, wherein a longitudinal axis of the valve shaft is parallelrelative to the inner and outer tubes.
 17. The shock absorber of claim1, further including a seal ring for sealing an annulus between theinner and outer tube.
 18. The shock absorber of claim 17, whereincommunication between the first gas chamber and the negative gas chamberis blocked by the seal ring when the piston reaches a predeterminedposition on the outer tube.